1. Field of the Invention
The present invention relates to a semiconductor device using a semiconductor thin film having a crystalline structure. Particularly, the present invention relates to a structure of an inverted stagger type thin film transistor (hereinafter abbreviated as a TFT). Moreover, the present invention relates to a structure of a semiconductor circuit, an electrooptical device, and an electronic equipment, each using the foregoing TFT.
Incidentally, in the present specification, the term “semiconductor device” indicates any device capable of functioning by using semiconductor characteristics. Any of TFTs, semiconductor circuits, electrooptical devices, and electronic equipments set forth in the present specification are contained in the category of the semiconductor device.
2. Description of the Related Art
Conventionally, a TFT is used as a switching element of an active matrix type liquid crystal display device (hereinafter abbreviated as AMLCD). At present, a market is occupied by products in which a circuit is constituted by TFTs each using an amorphous silicon film as an active layer. Particularly, as the structure of a TFT, an inverted stagger structure manufactured through simple steps is often adopted.
However, the performance of an AMLCD has been improved every year, and the operation performance (especially, operation speed) required for a TFT tends to become high. Thus, it becomes difficult to obtain an element having sufficient performance through the operation speed of a TFT using an amorphous silicon film.
Then a TFT using a polycrystalline silicon film (polysilicon film) instead of an amorphous silicon film has come into the limelight and the TFT having an active layer of the polycrystalline silicon film has been rapidly developed. At present, such TFTs have been partially made into products.
There are many publications as to the structure of an inverted stagger type TFT using a polycrystalline silicon film as an active layer. For example, there is a report “Fabrication of Low-Temperature Bottom-Gate Poly-Si TFTs on Large-Area Substrate by Linear-Beam Excimer Laser Crystallization and Ion Doping Method: H. Hayashi et al., IEDM95, pp829-832, 1995”, the disclosure of which is herein incorporated by reference, and the like.
Although the above report explains a typical example (FIG. 4) of an inverted stagger structure using a polycrystalline silicon film, the reverse stagger structure (so-called channel stop type) of such a structure has various problems.
First, since the entire of an active layer is as very thin as about 50 nm, impact ionization occurs in a contact portion between a channel formation region and a drain region, so that deterioration phenomena such as hot carrier injection strikingly appear. Thus, it becomes necessary to form a large LDD region (Light Doped Drain region).
The control of the LDD region becomes the most important problem. In the LDD region, the control of the concentration of impurities and the length of the region are very delicate, and especially, the control of the length becomes a problem. At the present circumstances, although a system in which the length of the LDD region is regulated by a mask pattern is adopted, if the degree of fineness is progressed, a slight patterning error causes a large difference in TFT characteristics.
The dispersion of sheet resistance of the LDD region due to the dispersion of the film thickness of an active layer also becomes a serious problem. Moreover, the dispersion in taper angles and the like of a gate electrode also may cause the dispersion of effects of the LDD region.
Further, a patterning step is required to form the LDD region, which directly causes manufacturing steps to increase and throughput to lower. According to the reverse stagger structure set forth in the above-mentioned report, it is expected that at least six masks (until formation of source/drain electrodes) are required.
As described above, in the reverse stagger structure of the channel stop type, the LDD regions must be formed at both sides of a channel formation region in a plane in a lateral direction, so that it is very difficult to form the LDD regions with reproducibility.